[BREAKING] Provide runtime environment systemd services from munix

These services evolve as munix evolves, so they should not be part of
the system closures themselves. Mount them into /run/systemd instead.

(Yes, making /run/systemd/system a symlink to RO files is unfortunate,
 that could be changed in the future. FS prep code is annoying too..)

devShells/default.nix

@@ -10,6 +10,7 @@
   libkrun,
   muvm,
   sidebus-broker,
+  wl-cross-domain-proxy,
   pkgs,
 }:
 
@@ -32,6 +33,7 @@ mkShell {
       passt
       bubblewrap
       sidebus-broker
+      wl-cross-domain-proxy
     ]
     ++ (with pkgs; [
       meson